PHA is a polyester-type compound which accumulates within a microorganism as a carbon source when the microorganism has excess sources of carbon available with a corresponding lack of nutrients, such as phosphorus, nitrogen, magnesium and oxygen. Since this PHA has physical properties similar to synthetic polymers originated from petroleum, while showing complete biodegradability, it is being recognized as a substitute for prior synthetic plastics.
Generally, PHA is divided into short-chain-length PHA (SCL-PHA) with a small number of carbon atoms and medium-chain-length PHA (MCL-PHA) with a relatively large number of carbon atoms. Since MCL-PHA has more similar physical properties to synthetic polymer than SCL-PHA, studies on generally MCL-PHA are being actively conducted. To produce PHA in a microorganism, different enzymes are required: enzymes capable of converting metabolites of the microorganism into PHA monomers, and a PHA synthase capable of synthesizing a PHA polymer from the PHA monomers. The enzymes known to provide the PHA monomers include (R)-specific enoyl-CoA hydratase found in Aeromonas sp. microorganisms and Pseudomonas sp. microorganisms, and 3-ketoacyl-ACP reductase found in E. coli and Pseudomonas sp. microorganisms (Fukui et al., J. Bacteriol., 180:667–73, 1998; Tsuge et al., FEMS Microbiol. Lett., 184: 193–8, 2000; Taguchi et al., FEMS Microbiol. Lett., 176: 183–90, 1999; Ren et al., J. Bacteriol., 182:2978–81, 2000; and Park et al., FEMS Microbiol. Lett., 214:217–22, 2002).
Meanwhile, it was reported that a MCL-PHA synthase gene was cloned from a Pseudomonas sp. microorganism, and MCL-PHA could be synthesized using a recombinant microorganism transformed with this gene (Qi et al., FEMS Microbiol. Lett., 157:155–62, 1997; Qi et al., FEMS Microbiol. Lett., 167:89–94, 1998; Langenbach et al., FEMS Microbiol. Lett., 150:303–9, 1997; WO 01/55436; U.S. Pat. No. 6,143,952; WO 98/54329; and WO 99/61624). Furthermore, it was reported that the use of a recombinant E. coli, from which FadB among enzymes of fatty acid degradation pathway had been deleted, could also produce MCL-PHA (Langenbach et al., FEMS Microbiol. Lett., 150:303–9, 1997).
On the other hand, since it was reported that YfcX enzyme, homologous to FadB could provide the PHA monomers when the fadB gene-deleted recombinant E. coli was used (Snell et al., J. Bacteriol., 184: 5696–5705, 2002) it was predicted that new enzymes of providing the PHA monomers besides YfcX would exist. Because the use of the fadB gene-deleted recombinant E. coli can effectively produce MCL-PHA, efforts to discover the enzymes of providing the PHA monomers in the fadB gene-deleted recombinant E. coli were continued but there were no particular results up to now.
Accordingly, there was a continued need to discover the new enzymes of providing the PHA monomers in E. coli. 